1 Field of the Invention
The present invention relates generally to seismic signal processing and, more particularly, to apparatus and methods for improved interpretation of seismic data including seismic data related to identification of stratigraphic and lithological features.
2 Description of the Background
To locate valuable hydrocarbon deposits, numerous techniques have been developed for transmitting seismic wave energy into the earth's subterranean formations, recording the reflected seismic wave energy, and processing the recorded data. Analysis of 3-D seismic data is now used extensively worldwide to provide a detailed structural image of subsurface reservoirs. A typical 3-D arrangement may use hundreds of receivers arranged in a patched manner with the lines of receivers being orthogonal to the shot line direction. The reflected seismic wave energy is detected using sensors such as geophones or hydrophones and processed to produce signals or traces that have numerous properties related to the seismic wave energy, such as frequency, amplitude, phase, instantaneous envelope, and the like.
To process, as well as to improve, the quality of the recorded signals, various techniques are used, such as geometry selection, selection of common depth point gathers, wavelet shaping, velocity analysis--i.e., a series of steps before final migration puts the earth's geological boundaries at their correct position. Each of these steps involves making assumptions that may not result in optimal clarity of the final result generally due to unknown factors of the earth's subterranean features. For instance, the step of migration may involve making assumptions about velocity modeling techniques where the actual velocity in the relevant portion of the earth's volume may not be precisely known due to various or complex subterranean features.
During one part of the processing sequence, traces are typically added after time shifting so as to be stacked upon each other at common earth x, y positions in a manner that is aimed to provide a more reliable signal by reinforcing the information in the signal and reducing the random noise that tends to average out. In other words, the information in the traces tends to be reinforced as more traces are added together or stacked. Likewise, random noise such as surface noise and the like tends to be averaged out, as the same random noise will not typically be present at each sensor. Thus, as is well known to those of ordinary skill in the art, a seismic trace corresponding to a particular subsurface location is typically a composite trace resulting from stacking of numerous traces corresponding to that subsurface location and produced by detection of seismic waves by receivers having that subsurface location as their common depth point. The improved data is then used to identify and characterize geology and lithology of subsurface formations. However, in some cases noise may still be present in the stacked data that detracts from the geological information in the data.
Until recently, interpretation of such information has generally overlooked effective evaluation of seismic discontinuities, correlations, and differences--i.e., the coherence between the stacked seismic data signal traces. The overall effect of recognizing the advantages of the latter approach to interpretation is a greatly improved method for detecting important geological constituents such as faults, fractures, and subtle subterranean features.
Coherence analysis is discussed in U.S. Pat. No. 5,563,949, issued Oct. 8, 1996, to Bahorich et al., which is hereby incorporated herein by reference, wherein a method is disclosed for the exploration of hydrocarbons. The method as described in more detail in U.S. Pat. No. 5,563,949, and referred to hereinafter as coherence analysis, typically comprises such steps as obtaining a set of seismic signal traces distributed over a predetermined three-dimensional volume of the earth, dividing the three dimensional volume into a plurality of vertically stacked and generally spaced-apart horizontal slices, dividing each of the slices into a plurality of cells having portions of at least three seismic traces located therein, measuring the cross-correlation between another pair of traces lying in another vertical plane to obtain a cross-line value, and combining the in-line value and the cross-line value to obtain one coherency value for each of the cells and displaying the coherency values.
See also WO 97/33184, published Sep. 12, 1997, to Higgs et al., which discloses a method for identifying faults and stratigraphic features within seismic data without interpreter bias by processing data to identify the minimum difference between seismic traces. Large values of difference are plotted as display attributes for seismic reflection data interpretation for two-dimensional and three-dimensional seismic data. The large values of difference represent faults and stratigraphic features within the seismic data. Dip azimuth and dip magnitude attributes can also be generated and displayed.
Another patent, U.S. Pat. No.5,724,309, issued Mar. 3, 1998, to Higgs et al., discloses a method for utilizing instantaneous phase and derivatives of instantaneous phase as display and/or plot attributes for seismic reflection data processing and interpretation for two-dimensional and three-dimensional seismic data. The spatial frequency, dip magnitude, and dip azimuth attributes of the seismic events are calculated using the rate of change of instantaneous phase with space, instantaneous frequency and velocity, and displayed or plotted to assist interpreters in identifying fault breaks and stratigraphic features in the earth's subsurface.
Very generally, coherence analysis has involved comparing each trace or portion of a trace with adjacent traces, preferably in two different directions, and determining how well the traces correlate with each other. This correlation, similarity/dissimilarity, difference is preferably referred to herein as "the coherence." The coherence between traces may be determined in numerous different ways, such as those discussed in the above-cited references. If significant changes are found between traces or portions of traces--i.e., low coherence--then it is likely that a subsurface feature that included sharp changes such as a fault or fracture produced this change. The coherence is itself then used as a trace and is plotted so that analysis may be visually made. While coherence analysis has provided a great improvement in the ability to identify faults and lithographic features, it has been found that many faults, fractures, and the like still remain unidentified or missed for reasons that may often be due to complex subterranean effects including structural effects and fluids within the formations and/or various non-optimal steps in the processing sequence. Consequently, there remains a need for an improved method of seismic signal research that increases the likelihood of selection of optimal variables and, when using a coherence analysis, increases the likelihood of detecting relevant subterranean features. There is also a need to reduce noise in stacked data. Those skilled in the art have long sought and will appreciate the present invention, which addresses these and other problems.